1. Field of the Invention
This invention relates to well cementing operations, and more particularly to a method of restricting influx of formation gas into a wellbore during primary cementing of a wellbore penetrating a potentially hydrocarbon-producing subterranean formation. Restricting gas influx into the wellbore helps achieve a good cement job by reducing the likelihood that gas channels will form in the cement before it is completely set. Channeling allows cross-flow between zones, often requiring correction by expensive cement squeeze jobs before the wells can be efficiently put on production. Another problem that sometimes occurs in cementing operations is that water in the cement slurry sometimes flows out into the formation, causing dehydration of the cement slurry. Cement dehydration can result in premature bridging of the cement slurry and lead to channeling and/or weak cement.
It is common practice in drilling wellbores into potentially hydrocarbon-producing subterranean formations to extend a casing or liner down the wellbore through the potential producing zone and then to cement the casing or liner in place by pumping cement down the wellbore and up the annulus formed by the outside of the casing or liner and the borehole wall, at least over the portion of the wellbore extending through the potential producing zone. The cement is maintained in the annulus, usually at a balanced or slightly overbalanced pressure relative to the formation pressure, until the cement hardens. The well is then completed by installation of a production tubing and by perforating the casing or liner and the cement between the casing or liner and the formation to provide a fluid path between the formation and the production tubing.
In actual operations, it is sometimes difficult to maintain the desired balance of pressure, and an influx of formation gas before the cement hardens can result in channeling in the cement. Channeling is undesirable for several reasons, and usually requires a remedial cement squeeze job, with resulting delays and added expenses.
2. The Prior Art
Sodium silicate solutions have been used to form a barrier to water intrusion in wells, as described in U.S. Pat. No. 4,428,424. However, prior to this invention, no process was available to reliably control formation gas influx and at the same time prevent premature dehydration of cement.
European Patent Application No. 89302558.2 describes shale-stabilizing drilling fluid additives which are similar in many respects to the skin-forming materials useful in carrying out the process of the present invention.